Examples of an emission tomography apparatus include a PET (Positron Emission Tomography) apparatus. The PET apparatus reconstructs a tomographic image of a subject only when a plurality of detectors detects radiation (γ-rays) coincidentally generated due to annihilation of positive electrons (positrons) (i.e., only when radiation is counted coincidentally). In particular, the PET apparatus is also referred to as a “positron tomography apparatus”.
Examples of a PET apparatus include one as illustrated in FIG. 10A with a so-called “full-ring” type detector unit. The detector unit has various detectors arranged in a ring shape. The examples of a PET apparatus also include one as illustrated in FIG. 10B with a detector unit a part of which is open. The detector unit is so-called a “partial-ring” and a part thereof is missing. Examples of a PET apparatus with the “partial-ring” detector unit include a mammography PET apparatus configured to capture a reconstructed image of the breast. The mammography PET apparatus has a C-shape detector unit as the partial-ring detector unit as illustrated in FIG. 10C. The PET apparatus with various detectors not being arranged in a ring shape and thus having a blank area allows the detector unit to approach a subject flexibly depending on a size of the subject, thereby achieving flexible tomography. Such the apparatus is not limited to one illustrated in FIG. 10B or FIG. 10C.
Here, data of the γ-rays from the subject with radiopharmaceutical administered thereto is referred to as “emission data”. Data obtained for absorption correction by externally emitting radiation of the same type as radiopharmaceutical is referred to as “transmission data”. Examples of a projection image of the emission data include a sinogram. The sinogram has a longitudinal axis in an angle direction and a horizontal axis in a uniaxial direction of an orthogonal coordinate system (e.g., an x-coordinate in arrangement of detectors). Reconstruction processing is performed to the sinogram to capture a reconstruction image.
In recent years, a reconstruction method is mainly used for capturing a reconstruction image by performing reconstruction processing to data referred to as “list mode” (referred to as “list mode data” or “list data”). The data is detection event information of γ-rays (e.g., a detector number, a detection time, and energy of γ-rays) stored in time sequence. Such the reconstruction method is called “list mode reconstruction”.
The data collected in the radiation tomography apparatus contains scatter components of radiation. Then, an estimating technique of estimating the scatter components of radiation has been suggested. See, for example, Non-Patent Literature 1. The technique in the Non-Patent Literature 1 is called a “convolution-subtraction method”. In the method, superimposition integration is performed to projection data of the emission data for estimating the scatter components under the concept that the scatter components in the emission data have distribution with broad spread (space distribution with low-frequency components). The data used in the Non-Patent Literature 1 is obtained with a PET apparatus having a full-ring detector unit.    Non-Patent Literature 1: Dale L Bailey, “A Convolution-subtraction scatter correction method for 3D PET”, Phys. Med, Boil. 39 (1994) 411-424)